Numerous cysteine and serine proteases have been identified in human tissues. A "protease" is an enzyme which degrades proteins or peptides into smaller components. The terms "cysteine protease" and "serine protease" refer to proteases which are distinguished by the presence of a cysteine or serine residue which plays a critical role in the catalytic process. Mammalian systems, including humans, normally degrade and process proteins via a variety of mechanisms including the actions of cysteine and serine proteases. However, when present at elevated levels or when abnormally activated, cysteine and serine proteases are involved in pathophysiological processes.
For example, calcium-activated neutral proteases ("calpains") comprises a family of intracellular cysteine proteases which are ubiquitously expressed in mammalian tissues. Two major calpains have been identified: calpain I and calpain II. While calpain II is the predominant form in many tissues, calpain I is thought to be the predominant form in pathological conditions of nerve tissues. The calpain family of cysteine proteases has been implicated in many diseases and disorders, including neurodegeneration, stroke, Alzheimer's disease, amyotrophy, motor neuron damage, acute central nervous system injury, muscular dystrophy, bone resorption, platelet aggregation, cataracts and inflammation. Calpain I has been implicated in excitatory amino-acid induced neurotoxicity disorders including ischemia, hypoglycemia and epilepsy. The lysosomal cysteine protease cathepsin B has been implicated in the following disorders: arthritis, inflammation, myocardial infarction, tumor metastasis, and muscular dystrophy. Other lysosomal cysteine proteases include cathepsins C, H, L and S. Interleukin-1.beta. converting enzyme ("ICE") is a cysteine protease which catalyzes the formation of interleukin-1.beta.. Interleukin-1.beta. is an immunoregulatory protein implicated in the following disorders and diseases: inflammation, diabetes, septic schock, rheumatoid arthritis, and Alzheimer's disease. ICE has also been linked to the apoptotic cell death of neurons which is implicated in a variety of neurodegenerative disorders including Parkinson's disease, ischemia and amyotrophic lateral sclerosis (ALS).
Cysteine proteases are also produced by various pathogens. The cysteine protease clostripain is produced by Clostridium histolyticum. Other proteases are produced by Trpanosoma cruzi, malaria parasites Plasmodium falciparum and P. vinckei and streptococcus strains. Hepatitis A viral protease (HAV 3) C is a cysteine protease essential for processing of picornavirus structural proteins and enzymes.
Exemplary serine proteases implicated in degenerative disorders include thrombin, human leukocyte elastase, pancreatic elastase, chymase and cathepsin G. Specifically, thrombin is produced in the blood coagulation cascade, cleaves fibrinogen to form fibrin and activates Factor VIII; thrombin is implicated in thrombophlebitis, thrombosis and asthma. Human leukocyte elastase is implicated in tissue degenerative disorders such as rheumatoid arthritis, osteoarthritis, atherosclerosis, bronchitis, cystic fibrosis, and emphysema. Pancreatic elastase is implicated in pancreatitis. Chumase, an enzyme important in angiotensin synthesis, is implicated in hypertension, myocardial infarction, and coronary heart disease. Cathepsin G is implicated in abnormal connective tissue degradation, particularly in the lung.
Given the link between cysteine or serine proteases and various debilitating disorders, compounds which inhibit these proteases would be useful and would provide an advance in both research and clinical settings.